Method of obtaining maximum separability of organic matter from ash in coal extraction processes



March 15, 1966 v. L. BULLOUGH ETAL 3,

METHOD OF OBTAINING MAXIMUM SEPARABILITY OF ORGANIC MATTER FROM ASH INCOAL EXTRACTION PROCESSES Filed April 23, 1965 3 Sheets-Sheet 1 COALRECEIVING WATER CRUSHING AND DRYING SOLVENT MIXING L'GHT DIGESTION ICENTRIFUG CENTRIFUGE (souo BOWL) (DISC) SLUDGE COKER SOLVENT STILL L.PITCH SLUDGE COKE COKER GAS COKE TAR 74 fie/(imam INVENTORS.

March 15, 1966 v. BULLOUGH ETAL 3,240,566

METHOD OF OBTAINING MAXIMUM'SEPARABILITY OF ORGANIC MATTER FROM ASH INGOAL EXTRACTION PROCESSES Filed. April 23, 1963 5 Sheets-Sheet 5 ('WLV)BHFISSEIEId EIQI'IVE) M M INVENTORS.

United States Patent 3 240 566 METHOD OF OBTAININGMAXIMUM SEPARABIL- ITYOF ORGANIC MATTER FROM ASH IN COAL EXTRACTION PROCESSES Vaughn L.Bullough, Florence, Ala., and Wilburn C.

Schroeder, College Park, Md., assignors to Reynolds Metals Company,Richmond, Va., a corporation of Delaware Filed Apr. 23, 1963, Ser. No.275,094 6 Claims. (Cl. 23-209.9)

This invention relates to the production of high purity carbon of thetype employed in carbon and graphite electrodes, and it deals moreparticularly with a novel process for the removal of impurities frombituminous coal to produce a substantially ash-free carbon which can beused in the manufacture of carbon electrodes suitable for theelectro-metallurgical industries, especially in the aluminum industry.

In the production of aluminum metal by the electrolysis of aluminumoxide, there is employed a carbon anode conventionally made frompetroleum or coke-oven pitch coke. The carbon anode is consumed duringthe electrolysis, causing many of the impurities present in the carbon,such as silicon, vanadium and iron, to pass into the aluminum.

Generally, a purchase specification of 1% total ash is imposed on cokeused in the manufacture of electrodes for the aluminum industry. Anauxiliary specification of less than 2% sulfur in the calcined coke isalso imposed. This severely limits the raw materials that can be used inthe manufacture of electrodes, and suitable materials heretofore havebeen confined to coke produced from certain grades of petroleumresiduum, coke produced by carbonization of coal tar or coke-oven pitch,and coke produced by the carbonization of gilsonite. There are reportedinstances where cokes of marginal purity have been produced from certainseams of coal by froth floatation and chemical cleaning, but there areno major seams of coal in the United States that can be cleanedadequately by froth flotation to produce coke that will meet thespecification as to ash content of anodes for use in the aluminumindustry. Chemical cleaning of coals, for example, with mineral acids,is an expensive procedure which limits its application to emergencies;and few seams of coal are available which are amenable to this process.

Because of its relatively high purity and availability, petroleum cokehas generally been the material of choice for the preparation of anodesof the type heretofore discussed, but not all petroleum crudes yield aresiduum of satisfactory quality for these purposes. In fact, theavailability of crudes which yield a satisfactorily low sulfur andvanadium content for the production of carbon of desired purity is solimited that sources of pure carbon for future expansion of domesticaluminum production are uncertain.

An advantage of the present invention resides in the production ofcarbon having virtually no vanadium and sufficiently low sulfur for usein the aluminum industry.

It is an object of the present invention to provide a method for theeconomical production of anode quality coke from coal.

It is another object of the invention to provide a method for thesolution of raw coal and the separation of ash from said solution,employing a self-sustaining regenerative solvent system.

It is a further object of the invention to provide a method for thesolution of raw bituminous coal, employing a solvent derived from saidcoal, and coking the coal solution, whereby electrode quality coke canbe obtained with the coal itself serving as substantially the sole rawmaterial.

These and other objects will appear from the ensuing description.

The fundamental problem in the conversion of coal into virtuallyash-free coke is that of effective separation of the ash and othermineral components of the coal. The ash-forming components comprise twogroups (a) inherent ash, constituting the mineral materials whichoccurred in the living plant and which became incorporated in the coalupon coalification, and (b) extraneous ash, including materialcodeposited with the plant materials originally, or which hassubsequently found its way into coal seams. Inherent ash contains iron,phosphorus, sulfur, calcium, potassium, and magnesium, and other plantnutrients. The extraneous ash may include common minerals such askaolinite, pyrite, and calcite, clay and shale.

Among known methods for separation of the ash from coal are those basedupon physical removal, such as floatsink, and froth flotation. Amongchemical methods, there have been proposed treatments with mineralacids, and with alkalis. These methods have not resulted in providing acoke residue sufficiently low in ash to meet the specifications forcarbon electrodes, or have been too expensive for commercial practice.

Coal solution processes have also been proposed for ash removal fromcoal. These are based upon the principle that coal, and particularlybituminous coal, is a complex mixture of polymeric molecules of unknownstructure, which are present together with fusain or mineral charcoal.It is known that certain coal-based hydrocarbons act as solvents forsome or all of the polymeric molecules, such solvents including, forexample, creosote oil and some of its higher hydrocarbon constituents,such as anthracene oil and phenanthrene. A method of coal treatment ofthis type is disclosed in Rose et al., Patent 1,925,005 (1933). Lahariet al. (India Patent 49,729, December 14, 1954) disclose the separationof fine particles of ash from solutions of coal in solvents by settling,filtration or centrifuging. However, the solutions produced by themethods of Rose et al. and Lahari et al. are highly viscous, and containlarge aggregates of colloidal coal particles with the fine ash soentrained that ordinary separation methods are unsuccessful.

In accordance with the invention it has been found that the solubleconstituents of coal, particularly bituminous coal, can be dissolved inan aromatic hydrocarbon oil of relatively low viscosity and boilingrange, under superatmospheric pressure, and at a temperature maintainedat above the temperature of maximum solubility of the coal constituents.

The aromatic hydrocarbon oil which is advantageously employed for thesolution treatment of coals, in accordance with the invention, ispreferably a lower boiling fraction obtained by the fractionaldistillation of coal tar or coke oven tar. Preferably the fraction is acreosote type fraction from which light oils have been substantiallyremoved, i.e. benzene, toluene, and mixed xylenes, so that the creosotetype oil has an initial distillation temperature of about 180 C. atatmospheric pressure. The creosote type extraction oil which has beenfound suitable for the purposes of the invention may, however, dependingupon the selection of operating conditions, include high boilinganthracene oil fractions, and in fact, it is within the contemplation ofthe invention that any make-up to the process necessitated by loss orremoval from the system of the solvent-valuable lower boiling creosotetype oil fractions, can be supplied from relatively inexpensive heavyresidue creosote oil.

Thus, in accordance with a first aspect of the invention, bituminouscoal can be extracted under critical and carefully controlledconditions, up to a solubility of more than 80%, in an aromatic liquidhydrocarbon creosote oil type solvent having a relatively low viscosityand a low initial boiling point of about 180 C. Moreover, as will bedescribed more fully below, this aromatic liquid hydrocarbon extractantcan be generated in the sustained operation of the process, so thatbituminous coal becomes the only raw material needed in the process.

For a better understanding of the invention and its various objects,advantages, and details, reference is made to the accompanying drawings,which are exemplary of the present preferred embodiments thereof, andindicate typical operating conditions. In the drawings:

FIG. 1 is a flow sheet of the process of the invention, with theprincipal steps thereof illustrated in diagram form.

FIG. 2 is a plotted series of curves graphically illustrating the effectof temperature on the amount of coal dissolved in the solvent employedand the purity of the coke produced therefrom.

FIG. 3 is a typical heating curve for the digestion of coal inaccordance with the invention.

In the practice of the first, or extraction aspect of the invention,there is selected a suitable bituminous coal, or other carbonaceousmaterial of similar composition. Preferably freshly mined coal is used,and the raw coal is crushed and pulverized to optimum particle size.Examples of suitable bituminous coals include Alabama and Kentucky highvolatile types. Coals of this type may analyze, for example, on theas-received basis, from 3545% volatile matter, 7284% carbon, and 2-10%ash, depending upon quality.

The coal is advantageously crushed and pulverized, then preheated, ifdesired, to reduce its moisture content. The crushing may be carried outin conventional equipment, for example, an air-swept ball mill with airclassifier, and the moisture reduced by a flow of heated flue gasthrough the ball mill, to a level of about 1%.

The crushed and dried coal is then fed to a suitable mixing tank andmixed with a suitable amount of a creosote type extraction oil of thecharacter described, or with an extraction oil fraction generated fromwithin the process system. The coal extraction oil mixture is thentransferred to a pressure digester. Alternatively, the coal andextraction oil can be fed directly into the pressure vessel, whichserves both as mixer and digester. The pressure vessel comprises anautoclave equipped with heating elements, a cooling coil, a stirrer, andpressure and temperature indicating devices.

The finely divided coal is heated in the digester with the solvent at atemperature slightly in excess of the temperature required to achievemaximum solubility of the coal constituents in the extraction oil,preferably at a temperature from about 5 to about 50 C. in excess of themaximum solubility temperature. Where the coal and oil are premixed, themixing may be carried out at a temperature of about 50 C. if the mixingtank is open, up to about 150 C. when the tank is closed.

The mixture of coal and extracting oil in the digester is brought to atemperature suitable for maximum solubility, as previously indicated.This temperature will generally lie in the range from about 380 C. toabout 450 0., depending upon the type of coal. The pressure in theextraction vessel at these temperatures will rise to about 750 to 2000lbs. per sq. in. gage. The mixture is stirred at a velocity sufiicientto maintain the solids in suspension. The mixture is held at theextraction temperature for a period of time from about 2 to 60 minutesbut a retention time of 10 to 30 minutes is preferred. The temperatureof the mixture is then reduced by cooling to between about and about 220C. Any gaseous or liquid hydrocarbon material present having a boilingtemperature below 160-220 C. is distilled off, and collected for use asfuel or as a valuable light oil byproduct of the process. The charge isthen transferred to to a centrifuging system, described below, forseparation of insolubles.

The weight ratio of extracting oil to coal may range from about 1:1 toabout 6: 1, but is preferably maintained between about 2.511 to 4:1.

The temperature to which the coal and extracting oil mixture is heatedhas been found to have a marked influence in the purity of the carbon orcoke ultimately obtained. As previously mentioned, this temperature mustvlJe above that of maximum solubility of the coal constituents toproduce a final carbon or coke which is substantially ash-free. It isthought that finely disseminated particles of ash in the hot extractionmixture at these elevated temperatures may act as nucleating sites forcarbonization, and thus some material which would otherwise contributeto ash content in the final carbon product is trapped Within a partiallycarbonized matrix and removed by subsequent processing.

In accordance with a second aspect of the invention, there is provided anovel method of purification of the extraction mixture of digested coalsolution, to separate therefrom a clarified coal solution, and a residueof undigested coal, carbon, and mineral ash. It has been found that theseparation of the digested mixture of coal and extract is best carriedout by a two-stage centrifuge treatment. The digested mixture from thepressure vessels, after removal of gas, volatile liquids, and cooling to160- 220 C., as previously described, is fed as a steam, in a firststage, to a disc-type centrifuge. The overflow from this centrifuge,which is the clarified coal solution, is transferred to heated tanks tobe held for distillation and coking The underflow, which contains about20% solids comprising undigested coal, 'carbon or charcoal, and mineralash, is fed, in a second stage to a solid bowl centrifuge where it isseparated into an overflow which is cycled back to the disc centrifugefor further clarification The underflow comprises a sludge containingfrom 40-50% solids which is discarded from the system. As mentioned,both centrifuge stages operate at about 160-220" C.

The final step of the process of the invention comprises thecarbonization of the purified coal solution discharged from the disctype centrifuge. This step converts the purified coal solution partly toa distillate and partly to a solid residue of carbon or coke. The amountof coke produced will generally range from about 13% to about 15% byweight of the amount of coal solution charged to the coking means. Thecarbonization of the coal solution can be carried out by anyconventional type coking apparatus such as, for example, rotating drum,delayed or fluid cokers of the type used in the carbonization of coal,coal tar pitch, or petroleum residue, or in by-product coke ovens. Ifthe latter are used, it (is desirable to subject the coal solution to apreliminary distillation or topping step to remove all possible solventbefore charging the coke oven. Coking temperatures will generally lie inthe range of 550700 C.

The distillate, which constitutes from about 7585% of the coal solutioncharged comprises a creosote type aromatic hydrocarbon containing liquidwhich meets the requirements previously disclosed for a suitable coalsolvent. Accordingly when the process is in full operation thisdistillate serves as the solvent for the treatment of the raw coal andit is necessary only to add make-up heavy residue creosote oil, asindicated previously to offset process losses. Hence the novel processof the invention is virtually completely self-sustaining in that therequired coal solvent is obtained from the coal itself in the operationof the process, so that raw coal is substantially the only raw materialwhich need be employed.

The following table shows the properties of cokes obtained from Kentuckyand Alabama c-oal after calcining the coke to 2450 F., in comparisonwith a typical petroleum coke:

TABLE I Properties of cakes from Kentucky coal and Alabama coal aftercalcining to 2450 F.

1 In ohms/inch/sq. inch.

The coke manufactured from bituminous coal in accordance with theinvention Was found to be Well suited for use in anodes of aluminareduction cells of both Soderberg and pro-bake types, performing as wellas or better than the petroleum coke used in regular anodes.

The following examples serve to illustrate the invention but are not tobe regarded as limiting.

EXAMPLE 1 100 lbs. of high volatile Western Kentucky bituminous coal(volatile matter 40.5%, ash 9.9%, sulfur 3.1%) was pulverized to minus48 mesh particle size and introduced into a heated stirred autoclavetogether with 300 lbs. extraction oil having an initial boiling point of180 C. and the mixture was heated at 400 C. and at a pressure of 1300p.s.i.g. for 30 minutes. The reaction mixture was cooled to 200 C. andcentrifuged at that temperature in a disc-type centrifuge equipped forcontinuous solid discharge. The purified coal solution constituting thecentrifuge overflow, upon coking, in a rotary steel drum at atemperature of about 900 F., was found to yield a coke which contained0.42% ash.

EXAMPLE 2 The treatment described in Example 1 was repeated using anamount of extraction oil equal to twice the weight of the coal. Thecalcined coke obtained from the purified coal solution contained 0.26%ash.

EXAMPLE 3 The process of Example 1 was repeated employing three timesthe weight of the coal of an extraction oil distillate produced from thecoking of the coal solution of Example 1. The product of the disccentrifuge, when coked, yielded a coke containing 0.39% ash.

EXAMPLE 4 100 lbs. of high volatile Alabama bituminous coal (35.4%volatile matter, 3% ash) was treated with three times its weight ofextraction oil having an initial boiling point of 180 C. by heating in astirred autoclave at 400 C. for 30 minutes at 1000 p.s.i.g. The coalsolution mixture was centrifuged in a disc-type centrifuge at 200 C. andthe purified solution was coked in a rotating steel drum at about 900 F.and the resultant coke was calcined at about 2400 F. in an inductionfurnace. The resulting coke analyzed 0.33% ash. When prepared in asuitable aggregate, the coke produced test electrode specimens of thefollowing properties when compared to typical petroleum coke treated incomparable manner:

Petroleum Coal Coke Apparent Density, gJcc 1. 50 1. 54 Resistivity,ohms/m./mm. 59. 4 58.0 Compression strength, lbs/in. 7,088 7, 564

EXAMPLE 5 A purified solution of the same coal as used in Example 4 wasprepared in a continuous digestion operation, then coked in a slot-typeby-product coke oven and calcined to about 1100 C. Samples from carbonpaste prepared from this coke for test in a commercial Soderberg cellwere found to yield electrode specimens having the following properties:

Pctroleum Coal Coke Coke Apparent density, g./cc 1. 56 1. 60Resistivity, ohms/m./mm. 61. 5 51. 5 Compression strength, lbs/in. 5,750 8,

EXAMPLE 6 The same digested coal solution as in Example 5 was fed at 200C. to a commercial size tar decanter centrifuge of the disc-type,equipped for continuous discharge of solids. The underflow stream fromthis centrifuge, containing essentially all of the mineral matter,undissolved coal, and mineral charcoal was fed to a solid bowlcentrifuge with continuous solids discharge features. The cakedischarged from the machine was approximately 50% solids. The overflowfrom the solid bowl machine was recharged to the disc-type centrifugefor further purification. This procedure was found to give the maximumpurity and minimum loss of product from the system drag-out with theundesirable solids, and the highest centrifuge capacity per unitmachine.

What is claimed is:

1. Method for the preparation of substantially ash-free carbon suitablefor electrodes from bituminous coal, comprising the steps of:

(a) forming a mixture of a finely divided bituminous coal and from about1 part to about 6 parts by weight of the coal of an aromatic hydrocarboncreosote type extraction oil having an initial boiling point of about180 C.;

(b) heating said mixture for a period of from about 2 to about 60minutes at a temperature from about 5 C. to about 50 C. in excess of thetemperature required to achieve maximum solubility of the coalconstituents in the oil and at a pressure from about 750 to about 2000p.s.i.g. to attain maximum separation of soluble coal constituents frominsoluble residue;

(c) reducing the temperature of the mixture to between about 160 C. andabout 220 C.;

(d) separating the resulting solution at between about 160 C. and about220 C. from the insoluble residue of the coal to form a clarifiedcarbonizable substantially ash-free coal solution; and

(e) distilling and carbonizing said coal solution to recover extractionoil and a residue of substantially ash-free carbon therefrom.

2. Method of forming a substantially ash-free bituminous coal solution,comprising the steps of:

(a) forming a mixture of a finely divided bituminous coal and from about1 part to about 6 parts by weight of the coal of an aromatic hydrocarboncreosote type extraction oil having an initial boiling point of about180 C.;

(b) heating said mixture for a period of from about 2 to about 60minutes at a temperature from about C. to about 50 C. in excess of thetemperature required to achieve maximum solubility of the coalconstituents in the oil and at a pressure from about 750 to about 2000p.s.i.g. to attain maximum separation of soluble coal constituents frominsoluble residue;

(c) reducing the temperature of the mixture to between about 160 C. andabout 220 C. and

(d) separating the resulting solution at between about 160 C, and about220 C. from the insoluble residue of the coal to form a clarifiedcarbonizable substantially ash-free coal solution.

3. The method of claim 1 in which the coal solution is separated fromthe insoluble residue of the coal by centrifuging at between about 160C. and about 220 C.

4. The method of claim 3 in which the centrifuging is performed in twostages, the sludge discharged from the first stage being fed to thesecond stage.

8 5. The method of claim 3 in which the centrifuging is performed in twostages, the effluent from the second stage being combined with the feedto the first stage.

6. The method of claim 1 in which the recovered extraction oil isrecycled to the mixing step (a).

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESCampbell et al.: Coal as a Source of Electrode Carbon in AluminumProduction, Bur. Mines Rep. of Investigations 5191, US. Dept. Int.,1956, pp. 10-13 and 3251.

Warnes: Coal Tar Distillation, 3rd E., pp. 56-58, Van Nostrand Co., NewYork (1924).

MORRIS O. WOLK, Primary Examiner.

2. METHOD OF FORMING A SUBSTANTIALLY ASH-FREE BITUMINOUS COAL SOLUTION,COMPRISING THE STEPS OF: (A) FORMING A MIXTURE OF A FINELY DIVIDEDBITUMINOUS COAL AND FROM ABOUT 1 PART TO ABOUT 6 PARTS BY WEIGHT OF THECOAL OF AN AROMATIC HYDROCARBON CREOSOTE TYPE EXTRACTION OIL HAVING ANINITIAL BOILING POINT OF ABOUT 180*C.; (B) HEATING SAID MIXTURE FOR APERIOD OF FROM ABOUT 2 TO ABOUT 60 MINUTES AT A TEMPERATURE FROM ABOUT5*C. TO ABOUT 50*C. IN EXCESS OF THE TEMPERATURE REQUIRED TO ACHIEVEMAXIMUM SOLUBILITY OF THE COAL CONSTITUENTS IN THE OIL AND AT A PRESSUREFROM ABOUT 750 TO ABOUT 2000 P.S.I.G. TO ATTAIN MAXIMUM SEPARATION OFSOLUBLE COAL CONSTITUENTS FROM INSOLUBLE RESIDUE; (C) REDUCING THETEMPERATURE OF THE MIXTURE TO BETWEEN ABOUT 160*C. AND ABOUT 200*C. AND(D) SEPARATING THE RESULTING SOLUTION AT BETWEEN ABOUT 160*C. AND ABOUT220*C. FROM THE INSOLUBLE RESIDUE OF THE COAL TO FORM A CLARIFIEDCARBONIZABLE SUBSTANTIALLY ASH-FREE COAL SOLUTION.